The effects of the relative velocities of moving objects on their apparent relative size and depth were investigated with the aid of square patterns generated on a CRT display by a microcomputer. The observer matched the apparent sizes of squares, arranged in two or more rows that moved with different velocities, and made judgments of the apparent relative depth of the rows. In many conditions, squares moving more slowly were perceived as larger in size than those moving faster, regardless of the kind of depth responses, but in some conditions which contained strong depth cues the size responses seemed to be affected by perceived depth. The size-change effect of moving objects is discussed from the viewpoint of size-depth relationship.